This invention relates generally to the plasma processing of semiconductor wafers and more particularly to methods and apparatus for plasma processing semiconductor wafers in a reactive ion etch mode.
Integrated circuits (ICs) are fabricated on semiconductor wafers by subjecting the wafers to a precise sequence of processes. These processes can include, but are not limited to, epitaxial deposition, lithographic patterning, chemical vapor deposition, sputter deposition, ion implantation and etch processes.
There is a seemingly inexorable trend in the IC industry to produce more powerful integrated circuits by packing ever greater numbers of active and passive devices into each integrated circuit. This is typically accomplished by both reducing the sizes of the devices within an IC and by arranging the devices more closely together.
As IC devices become smaller and more densely packed they also become more susceptible to damage during the aforementioned processing steps. For example, when the minimum feature size (such as a line width) of an IC reaches about 1 micron, the devices of the integrated circuit may be susceptible to damage if exposed to voltage levels over 200 volts. Since it is not unusual for conventional semiconductor processing equipment, such as a reactive ion etch (RIE) system, to develop considerable voltage levels during operation, steps must be taken to prevent damage to the devices of the integrated circuits.
In a reactive ion etch system a process gas is released into a process chamber and a radio-frequency (RF) power source is coupled to a cathode located within the chamber to create a plasma from the process gas. A semiconductor wafer can be supported by the cathode and positive ions formed within the plasma can be accelerated to the surface of the wafer to provide a very anisotropic etch of the wafer's surface. Conventional RIE systems have been operated at a number of frequencies including a low frequency range from about 10-400 kilohertz and a high frequency range from about 13-40 megahertz.
Both ions and electrons within a plasma are accelerated in systems operated in the low frequency range of 10-400 kilohertz creating the risk of potential damage to IC devices caused by the impact of heavy, high-energy ions against the surface of the wafer. In high frequency operation in the 13-40 megahertz range a steady state cathode sheath is formed near the cathode which typically develops a magnitude of over 1000 volts at 1 kilowatt of power. As mentioned previously, voltages of this magnitude can be very damaging to high-density IC circuitry. In contrast, systems operated in the microwave range of about 900 megahertz to 2.5 gigahertz, such as electron cyclotron resonance (ECR) systems, have sheath voltages so low that an auxiliary bias on the cathode is often required to provide commercially useful etch rates.
Of the frequency range choices, the high frequency range of 13-40 megahertz is most often employed in plasma etch systems. By far the most popular choice for an RIE system operating frequency is the ISM (industry, scientific, medical) standard frequency of 13.56 megahertz. However, the potentially damaging sheath voltages of such systems limits their usefulness in performing certain sensitive etch processes, such as a polysilicon over silicon dioxide ("oxide") etch.
The cathode sheath voltage can be reduced by the use of magnetic confinement techniques such as those disclosed in U.S. Pat. No. 4,842,683 entitled "Magnetic Field-Enhanced Plasma Etch Reactor" of Cheng et al. which teaches the use of a rotating magnetic field above the surface of a wafer having magnetic flux lines substantially parallel to the wafer surface. The magnetic field of Cheng et al. decreases the cathode sheath voltage 25-30 percent, i.e. to about 700 volts, while it increases the etch rate by as much as 50 percent.
A problem with a magnetic enhancement system as disclosed by Cheng et al. is that the electric (E) field created within the cathode sheath is substantially perpendicular to the wafer surface and is therefore at substantially perpendicular to the applied magnetic (B) field. The E.times.B force created by the interaction of these two fields causes the well-known electron/ion drift effect, which is a major source of etch non-uniformity in magnetically enhanced RIE systems. The aforementioned rotation of the magnetic field reduces, but does not eliminate, etch non-uniformity by averaging the effects of the electron/ion drift over the surface of the wafer.
Another problem encountered with the system of Cheng et al. is that, even with magnetic confinement, a cathode sheath voltage at about 700 volts is still too large to avoid damaging IC devices during certain types of processes. Unfortunately, since the electron/ion drift is caused by the E.times.B force, raising B to lower the cathode sheath voltage will increase the electron/ion drift effect a corresponding amount. In consequence, the system of Cheng et al. cannot reduce the cathode sheath voltage much below 700 volts by further increasing the B field strength without causing an unacceptably high etch non-uniformity over the surface of the wafer.
One approach to reducing the cathode sheath voltage to acceptably low levels is disclosed in parent application U.S. Ser. No. 07/559,947, filed Jul. 31, 1990, of Collins et al. and entitled "VHF/UHF Reactor System", the disclosure of which is incorporated herein by reference. It is known that the cathode sheath voltage is a function of the RF impedance (Z.sub.RF) of the plasma which is given by the following relationship: EQU Z.sub.RF =R-jx
where R is the resistive component of the plasma impedance and x is the reactive component of the plasma impedance. Therefore, an increase in RF frequency causes a decrease in Z.sub.RF and a consequent reduction in the cathode sheath voltage. Collins et al. teach that operating a RIE system at VHF/UHF frequencies from about 50 megahertz to about 800 megahertz will result in lower cathode sheath voltages resulting in a softer, less damaging etch processes.
While the system of Collins et al. performs very well, it suffers from the drawback that it requires a variable frequency R.F. power supply or multiple frequency R.F. sources which, for the required power and frequencies, are very large and very expensive. Also, their impedance matching network was, to some extent, a compromise over the range of operating frequencies, resulting in less than optimal impedance matching at any one frequency within the range.
In consequence, there was heretobefore an unsatisfied need for a plasma processing system in which the cathode sheath voltage could be controlled both inexpensively and effectively.